1. Field of the Invention
The invention relates to a power converting device, more particularly to a power converting device which adopts a synchronous rectifier circuit.
2. Description of the Related Art
In a conventional forward power converter, a main switch is disposed at the primary side thereof, and a rectifier circuit which is composed of rectifier diodes is disposed at a secondary side thereof. However, a considerable energy loss is incurred from conduction of the rectifier diodes. Therefore, referring to FIG. 1, a synchronous rectifier switch (MOS transistor switch) Q2 is usually adopted to replace the rectifier diodes at the secondary side of a transformer T1 of an existing forward power converter, and a synchronous rectifier controller 6 is used to control conduction and non-conduction of the synchronous rectifier switch Q2.
The existing synchronous rectifier controller 6 is able to operate in one of a discontinuous conduction mode (DCM) and a continuous conduction mode (CCM) in response to a requirement for different loads of the forward power converter. For example, a conventional synchronous rectifier controller of a model number SG6203 is able to detect the voltage drop at the synchronous rectifier switch Q2, and to detect a magnitude of a current associated with the synchronous rectifier switch Q2, so as to control the synchronous rectifier switch Q2 to become non-conductive upon detecting that the magnitude of the current drops to zero. However, the aforementioned control method is only suitable for the DCM.
In the CCM, since the synchronous rectifier switch Q2 is required to be switched to non-conduction before an output current (i.e., a current flowing through the synchronous rectifier switch Q2) drops to zero, the synchronous rectifier controller of the model number SG6203 is incapable of operating in the CCM by means of detecting the magnitude of the current through the synchronous rectifier switch Q2. Therefore, the SG6203 still requires an RC (resistor-capacitor) trigger to forcibly turnoff the synchronous rectifier circuit Q2. However, owing to the RC time constant of the RC trigger, the aforementioned solution is not suitable for a situation where a load varies rapidly.
Furthermore, a conventional synchronous rectifier controller of a model number STSR30 utilizes an up counter and a down counter of a digital circuit to calculate most recent duty cycles of the main switch Q1 and the synchronous rectifier switch Q2. The most recent duty cycles thus calculated are adapted to serve as next duty cycles of the main switch Q1 and the synchronous rectifier switch Q2, such that the STSR30 is able to operate in the DCM and the CCM.
On the other hand, a conventional synchronous rectifier controller of a model number FAN6204 applies the volt-second balance principle, and calculates charge time and discharge time of a timing capacitor so as to control conduction times of the main switch Q1 and the synchronous rectifier switch Q2. Specifically, when the main switch Q1 is conductive, the timing capacitor is charged until the main switch Q1 becomes non-conductive, and subsequently, the timing capacitor starts to discharge, causing the synchronous rectifier switch Q2 to become conductive until the timing capacitor discharges completely. In this way, the FAN6204 is able to operate in the DCM and the CCM.
All of the aforesaid synchronous rectifier controllers of model numbers SG6203, STSR30, and FAN6204 utilize the charging and discharging of a capacitor for counting or for determining the conducting time and non-conducting time of switches. Nevertheless, the capacitor takes a response time to charge and discharge, such that when the load varies rapidly, for example, referring to FIG. 2, within a time interval t1 when the load varies from a heavy load to a light load, the synchronous rectifier controller may not catch up with switching changes of the main switch Q1 because it takes time for the capacitor to charge and discharge. The synchronous rectifier switch Q2 may thus not be turned off (i.e., non-conductive) in time, so that the main switch Q1 at the primary side may become conductive at the time the synchronous rectifier switch Q2 is not yet turned off. In this situation, the synchronous rectifier switch Q2 has to sustain an instantaneous high voltage generated by a secondary winding according to induction of an electromagnetic field from a primary winding. At the same time, since the synchronous rectifier switch Q2 is still conductive, when an output current ILO drops to zero, a reverse current Ir occurs as a result of an output capacitor Co discharging toward an output inductor Lo and the synchronous rectifier switch Q2, such that a voltage spike Vsp is generated between a drain terminal and a source terminal of the synchronous rectifier switch Q2 at the instant when the synchronous rectifier switch Q2 is turns off. The synchronous rectifier switch Q2 may be damaged by the voltage spike Vsp if voltage tolerance thereof is insufficient.
This reverse current Ir not only occurs when the load varies rapidly, but also occurs at the moment when the power converter is started up or shut down. When the power converter is started up or shut down, the output current ILO, may have a moment of zero current, such that the synchronous rectifier switch Q2 may be damaged by the voltage spike Vsp, if not turned off in time. A synchronous rectifier forward converter including a reverse current suppressor is disclosed in U.S. Pat. No. 7,589,982. The synchronous rectifier switch thereof is adapted to be turned off earlier than shutdown of the synchronous rectifier forward converter so as to eliminate or suppress the reverse current. However, the synchronous rectifier forward converter disclosed in the U.S. patent is also incapable of preventing generation of the reverse current under the condition that the load varies rapidly.